Space travel appears to have taken its toll on NASAastronauts. Those in space for more than a month have shown unexpected symptoms that could set back plans for longer deep-space missions.

Researchers scanned the eyes and brains of 27 astronauts who had spent an average of 108 days in space, either on space shuttle missions or aboard the International Space Station. In many of the astronauts, they found symptoms consistent with intracranial hypertension, a potentially serious condition that happens when pressure builds within the skull.

“NASA has placed this problem high on its list of human risks, has initiated a comprehensive program to study its mechanisms and implications, and will continue to closely monitor the situation,” Dr. William Tarver, chief of the flight medicine clinic at NASA’s Johnson Space Center told Fox News in a recent article.

A symptom of excess cerebral-spinal fluid around the optic nerve was found in 33 percent of the astronauts and a flattening of the back of the eyeball was found in 20 percent.

“The MRI findings revealed various combinations of abnormalities following both short and long-term cumulative exposure to microgravity also seen with idiopathic intracranial hypertension,” said Professor Larry Kramer, lead author of the study.

Odds are the study will have no effect on Virgin Galactic‘s plan to begin suborbital commercial flights this year.